This invention relates to improvements in or relating to the sampling of optical signals and, in particular, but not especially, a device and a method for sampling ultra-fast (ultra-high frequency) optical signals, such as C-band (3.7 to 4.2 GHz) signals, by achieving picosecond resolution.
The need to sample ultra-fast optical signals having ultra-short pulses is rapidly increasing, being of interest in a large number of applications, such as ultra-fast communications, biophotonics, sensing, large system synchronisation, dynamic characterisation and testing of new materials. To date, various optical sampling techniques have been proposed to sample ultra-fast optical signals.
One such technique is the Frequency Resolved Optical Gate (FROG). This is the only technique that can describe both amplitude and phase of an optical signal and does not require any synchronisation circuitry. However, it is a very complex technique and requires a long processing time. In particular, FROG requires a spectrometer to obtain the whole spectrum of the secondary signal obtained from the interaction of an optical signal to be sampled and its delayed replica. Consequently, the amount of data to be processed is large and can take a long time, making the sampling process very slow. Moreover, processing of the secondary signal involves the determination of an integral. However, in certain cases, the integral does not converge, preventing the processing of the secondary signal.
Other techniques require a clock recovery system to synchronise the pulses of the optical sampling signal with the optical signal to be sampled. If the repetition rate is very high, the development of a suitable clock recovery system for synchronisation is critical and can be very expensive. Furthermore, these techniques can typically only analyse limited time intervals, therefore they only allow one to resolve the eye-diagram of an optical data signal not the long bit sequences. Other optical sampling techniques carry out asynchronous sampling, using very high-speed electronics for the generation of a sampling gate. However, besides suffering from the disadvantage of requiring complex and costly electronics, the resolution is limited to tens of picoseconds.